Duchy of Teschen in the context of Duchy of Oświęcim

Cieszyn Silesia, Těšín Silesia or Teschen Silesia (Polish: Śląsk Cieszyński [ˈɕlɔ̃sk tɕɛˈʂɨj̃skʲi] ; Czech: Těšínské Slezsko [ˈcɛʃiːnskɛː ˈslɛsko] or Těšínsko [ˈcɛʃiːnsko] ; German: Teschener Schlesien or Olsagebiet) is a historical region in south-eastern Silesia, centered on the towns of Cieszyn and Český Těšín and bisected by the Olza River. Since 1920 it has been divided between Poland and Czechoslovakia, and later the Czech Republic. It covers an area of about 2,280 square kilometres (880 sq mi) and has about 810,000 inhabitants, of which 1,002 square kilometres (387 sq mi) (44%) is in Poland, while 1,280 square kilometres (494 sq mi) (56%) is in the Czech Republic.

The historical boundaries of the region are roughly the same as those of the former independent Duchy of Cieszyn. Currently, over half of Cieszyn Silesia forms one of the euroregions, the Cieszyn Silesia Euroregion, with the rest of it belonging to Euroregion Beskydy.

The Duchies of Silesia were the more than twenty divisions of the region of Silesia formed between the 12th and 14th centuries by the breakup of the Duchy of Silesia, then part of the Kingdom of Poland. In 1335, the duchies were ceded to the Kingdom of Bohemia under the Treaty of Trentschin. Thereafter until 1742, Silesia was one of the Bohemian crown lands and lay within the Holy Roman Empire. Most of Silesia was annexed by the King of Prussia under the Treaty of Berlin in 1742. Only the Duchy of Teschen, the Duchy of Troppau and the Duchy of Nysa remained under the control of the Bohemian crown and as such were known as the Duchy of Upper and Lower Silesia until 1918.

Bielsko [ˈbjɛlskɔ] (German: Bielitz, Czech: Bílsko) was until 1950 an independent town situated in Cieszyn Silesia, Poland. In 1951 it was joined with Biała Krakowska to form the new town of Bielsko-Biała. Bielsko constitutes the western part of that town.

Bielsko was founded by the Cieszyn Piast dukes in the late 13th century on the grounds of village later called Stare Bielsko (Old Bielsko), on the Biała River. It was first mentioned in a written document in 1312. Originally settled by Germans, it became the largest German-language center (Deutsche Sprachinsel Bielitz) in the Duchy of Teschen, and remained so until the end of World War II. In 1572 it gained autonomy as the Duchy (State) of Bielsko. During the 18th century a rapid development of textile industry occurred, and at the beginning of the 19th century more than 500 weavers worked in the town. After the 1920 division of Cieszyn Silesia between Poland and Czechoslovakia it became, despite the protests of local Germans, a part of Poland.